University of Maryland astronomer Jessica Sunshine, assistant principal investigator for NASA's $46 million EPOXI mission, said the 1.2-mile-long comet nucleus seems to be throwing off tons of gas and dust from its rough-looking ends, while accumulating smooth drifts of fine-grained material in lower terrain at the center.

"We have a lot of work to do to try to understand what's going on here. This is just spectacular," she said. The spacecraft was reassigned after UM's 2005 Deep Impact mission to Comet Tempel 1. It wraps up the Hartley 2 visit at year's end, and NASA is inviting scientists' proposals for a new target.

The images were transmitted from NASA's EPOXI spacecraft and displayed just after 11 a.m. on screens at the Jet Propulsion Laboratory in California. It is only the fifth time that a spacecraft has sent back close-up photos of a comet's nucleus. It is also the smallest of the five, barely a half-mile long.

The first close-up pictures were greeted with exclamations, cheers and applause in the JPL control center. The EPOXI mission is being led by scientists at the University of Maryland College Park, and principle investigator Michael A'Hearn.

"Thanks to all, Great Job!" A'Hearn told the JPL controllers.

Also on hand was Malcolm Hartley, the Australian astronomer who discovered the comet as a "smudge" on photographic plates in 1986. After the first high-resolution images were received, he began a circuit of the control room, shaking hands with everyone.

"It's absolutely awesome. I've been awed by everything that's happened in the last couple of weeks," he said.

JPL engineers reported the spacecraft made 199 successful images of the comet during the close pass, with no failures.

Thousands of images taken before, during and after the encounter will be transmitted to Earth later Thursday. The flyby was within a kilometer of the predicted path, about 435 miles from the comet.

The mission's goal is to capture imagery and data on the structure and composition of the comet and its halo of dust and gas. Scientists hope that a comparison of Hartley 2 data with information gathered from other comets will explain observed differences between them, and yield new clues to the conditions present during the formation of the solar system 4.5 billion years ago.

Discovered in 1986 by Australian astronomer Malcolm Hartley, the comet is the smallest of the five that have been visited by spacecraft from Earth. Its nucleus is believed to be only about three-quarters of a mile across.

"Despite that, it puts out more gas per minute," A'Hearn said. "It's a really active nucleus for its size."

Early images have revealed multiple jets of gas spewing from the comet as it spins in space.

"This is going to give us the most extensive view of a comet to date," said Tim Larson, the EPOXI project manager at NASA's Jet Propulsion Laboratory in California.

It's not easy. Mission planners had to steer EPOXI close enough to be able to see detail on the little comet, but not so close that it cannot swivel fast enough to keep the comet in its sights.

EPOXI scientists expect to collect 5,000 images. And no one will be more thrilled to see them than Hartley.

"It's a very special time for me," he said. "I couldn't imagine, 24 years ago, that I could be seeing something like this."